The present invention relates to a semiconductor laser element, a manufacturing technique thereof, and a semiconductor laser device.
A semiconductor laser device is widely used as an optical communication light source and an information device light source. The semiconductor laser device of various package structures are manufactured. Moreover, semiconductor laser elements (semiconductor laser chips) of various structures are manufactured so as to be built in the semiconductor laser device. Among them, a ridge-type semiconductor laser element is known.
The ridge-type semiconductor laser element, for example, has a structure having a semiconductor substrate made of a compound semiconductor, a first clad layer arranged on the main surface of the semiconductor substrate, an active layer arranged on the first clad layer, a second clad layer formed on the active layer, an etching stop layer formed on the second clad layer, a stripe-shaped ridge formed on the etching stop layer, and an insulation film arranged on the etching stop layer so as to cover the side and the top of the ridge. The ridge has a third clad layer formed on the etching stop layer and a contact layer formed on the third clad layer. The contact layer is electrically connected to a first electrode (such as an anode electrode) via an opening formed in the insulation film, and a second electrode (such as a cathode electrode) electrically connected to the semiconductor substrate is arranged on the rear surface of the semiconductor substrate opposite to its main surface.
The first electrode, for example, has a structure having a Ti film, a Pt film, and an Au film formed in this order from the insulation film side. The Ti film is formed mainly so as to suppress diffusion of atoms of the Au film into the ridge. The Pt film is formed mainly so as to enhance the attachment between the Ti film and the Au film. The Au film is formed mainly to suppress oxidization and lower the resistance.
It should be noted that the ridge-type semiconductor laser element is described, for example, in ITO Ryoichi and NAKAMURA Michiharu ed. “Handoutai Laser (Semiconductor Laser)”, Baifukan, 1991, Chapter 5 (KAYANE).
Moreover, the ridge-type semiconductor laser element using the Ti/Pt/Au structure for the first electrode is described, for example, in IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 46, NO. 8, August 1999 [High-Temperature, Low-Threshold Current, and Uniform Operation 1×12 Monolithic AlGaInAs/InP Strain-Compensated Multiple Quantum Well Laser Array in 1.5 micrometers].
In order to obtain a high reliability in the ridge-type semiconductor laser element, it is necessary to completely cover a clad layer exposed from the ridge side surfaces (two side surfaces located at opposing positions in the ridge width direction) with an insulation film. On the other hand, in order to reduce the contact resistance and the thermal resistance in the ridge, it is necessary to increase the contact area between the contact layer and the electrode. However, in the conventional ridge-type semiconductor laser element, an opening is formed in the insulation film on the ridge with an opening width (width in the same direction as the ridge width) narrower than the ridge width (width of the same direction as the direction intersecting the optical axis) and via this opening, the ridge contact layer is connected to the electrode. Accordingly, the contact resistance and the thermal resistance in the ridge are high.
Moreover, in the ridge-type semiconductor laser element using the Ti/Pt/Au structure in the first electrode, coverage of the side surface in the ridge X-direction (direction orthogonally intersecting the optical axis) by the insulation film greatly affects the reliability. For example, an insulation film of silicon oxide or the like has a barrier effect against diffusion of the Au. Accordingly, when the ridge X-direction side surface is completely covered with an insulation film, it is possible to prevent diffusion of Au coming into the ridge from the Au film of the first electrode through the ridge side surface even if the Ti film coverage at the ridge side surface is incomplete. However, when the ridge X-direction side surface is not covered completely, Au is diffused into the ridge from the Au film if the Ti film coverage at the ridge side surface is incomplete. Since Au is easily diffused with respect to crystal, the Au introduced into the ridge is further diffused with lapse of time and reaches a resonance region (light emission unit) arranged at a portion of the active layer immediately below the ridge. Since the diffusion region where Au is diffused absorbs light, the Au coming into the resonance region lowers ratio of conversion from current to light.
The Ti film is formed, for example, by deposition. Such a Ti film has a worse coverage at the ridge side surface as compared to the flat portion and the Ti film at the ridge side surface has a small thickness. Moreover, the thickness of the Ti film at the ridge side surface becomes thinner because of the production irregularities and process trouble.
Consequently, it is necessary to completely cover the ridge X-direction side surface with an insulation film. Especially Au is easily diffused into an InP layer than an InGaAs layer. Accordingly, the X-direction side surface of the third clad layer formed by InP should be covered completely with an insulation film.